1. Field of the Invention
The present invention relates to radio frequency identification (RFID) technology and, more particularly, to a method for conducting a label query in a RFID system and a reader to implement the method.
2. Background of the Invention
An RFID system basically comprises a reader and labels. Usually each label has a unique identification, and the labels can be attached to an object, such as a commodity, to identify the target object and can carry other relevant information. The reader sends a signal to the label or receives a signal from the label in an RF link by an antenna to read the identification and other relevant information of the target object from the label. There are two types of labels, i.e., read-only and read-write. For a read-write label, the reader can write relevant information to the label. Depending on the excitation method, labels can be classified into active RFID labels and passive RFID labels. An active label is powered by a battery to send signals to the reader actively. Without a battery, a passive label needs to obtain energy from the RF wave emitted by the reader for power supply. Therefore, the query process of the RFID system using passive labels is initiated by the reader.
Taking Electronic Product Code (EPC) global Class-1 Generation-2 standard EPC Radio-Frequency Identity Protocols Class-1 Generation-2 (C1G2) UHF RFID Protocol for Communications at 860 MHz-960 MHz Version 1.0.9 as an example, the basic operation process of the RFID system using passive labels is shown below. The reader loads information related to query at a certain radio frequency and sends it to the label. The label obtains energy from the radio frequency signal, and acquires information related to the query from the radio frequency signal received from the reader. Then, the reader sends a continuous-wave radio frequency signal to the label, and receives returned information from the label at the continuous-wave frequency, and the label returns information related to the label to the reader at the continuous-wave frequency through backscattering. The signal direction from label to reader is called a reverse link.
The RFID system transmits information through a radio signal. Consequently, the environment influences the performance of the radio link, thus influencing the operation performance of the system, such as reading rate and reading time. In an actual multipath environment, it is possible that not all passive labels can return information to the reader successfully due to poor performance of the reverse link of some passive labels. To solve this problem, in conventional systems several queries are usually conducted for labels which failed to return information in order to increase the reading rate of the system. However, in these queries, the reverse link uses a fixed link frequency (LF) to return information from the label to the reader, which cannot actively improve and influence the performance of the reverse link. Therefore, reliance is normally made simply on repeated readings to increase the reading rate.
In addition, in an environment with dense readers, the reverse link from passive label to reader may develop another problem. That is, as readers do not know the reverse link frequency selected by other readers, one reader may select a reverse link frequency that has already been used by another reader. At this point in time, the reverse link of some readers is blocked at this reverse link frequency, causing poor performance of the reverse link.
To solve the aforesaid problems, a method to conduct a label query in the RFID system and corresponding apparatus are needed to improve the successful reading rate on passive labels by improving the performance of the reverse link while balancing overall reading time.